my computer seems to be shutting down on me at times, i thought it was overheated but its not hot, im thinking its my power supply isnt good, but i was downloading and i opened a few programs and web pages, then the processor fan was making a loud whirling noise, typical noise but louder, then after about a half hour or so it just shuts down, and then it wont start up again, before i just left it alone for a few minutes and it came back on, but its not doing that now, any help?